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Chemistry of neutron stars modelled for first time

By David Shiga

A neutron star that siphons material from a partner has a solid crust of heavy elements topped by an ocean of lighter elements. Occasionally, the ocean erupts in a nuclear explosion (Illustration: C Horowitz et al/Indiana University)

Neutron stars are like layered candies, with different chemicals concentrated at different depths, reveal the first detailed computer simulations of the stars’ chemistry. If correct, this could affect the strength of any gravitational waves the stars emit, and may help explain the origin of the spectacular nuclear explosions seen tearing across their surfaces.

Neutron stars are incredibly dense remnants of supernova explosions, with a billion tonnes of matter packed into every cubic centimetre at their cores. Scientists study them as natural laboratories to learn more about the behaviour of matter under extreme pressures and temperatures.

Inside their cores, the pressure is so great that individual atoms are crushed out of existence, leaving only a dense liquid of neutrons. Surrounding the core is a crystalline crust, where the pressure is low enough for atomic nuclei to retain their individuality.

For some neutron stars, the crust is the outermost layer. But other neutron stars slowly siphon material from a companion star, and this material pools on top of the crust to form an outer liquid “ocean”. As material at the bottom of the ocean solidifies under pressure, the new material also adds to the crust.

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Scientists had previously assumed that the new crust would have the same chemical composition as the overlying ocean. But a new study led by physicist Charles Horowitz of Indiana University in Bloomington, US, suggests that metals such as iron solidify more readily to form the new crust, leaving lighter elements such as oxygen to build up in the ocean.

Mysterious concentration

To come to this conclusion, the researchers modelled what happened when a well-mixed selection of 17 elements was added to the outer layers of a virtual neutron star.

It is thought that a runaway nuclear reaction – involving carbon – in the outer ocean could explain the bursts. But this could occur only if the proportion of carbon there is very high – between 10 and 20%, and researchers have not been able to explain how it could get so concentrated.

‘Out of ideas’

Although carbon was not included in the model, the new work suggests it would behave like oxygen, which has a similar atomic weight, and remain in the outer ocean.

“The heavier stuff freezes, but the carbon doesn’t like to go in the solid, it likes to stay in the liquid,” Horowitz told New Scientist. “That could explain how there was enough carbon in the ocean to explode.”

Neutron star researcher Andrew Cumming of McGill University in Montreal, Canada, who was not a member of Horowitz’s team, agrees that this could be the solution to the puzzle.

“We’ve reached a situation in the last year or so where we ran out of ideas [to explain the bursts],” he told New Scientist. “This is exactly the kind of idea that might work.”

Space-time ripples

The chemical layering in neutron stars might also have important implications for whether the stars will emit detectable gravitational waves, hypothesised ripples in space-time that have yet to be detected.

Fast-spinning objects that are not perfectly symmetrical – such as neutron stars with bumps, or “mountains”, on their surfaces – are predicted to radiate away energy in gravitational waves. But no one knows how big any such bumps would be and therefore how strong the resulting waves would be.

“One question you can ask is how big a mountain you can have on a neutron star, and that depends on what the mountain is made of, its composition and its strength,” Horowitz says. He says the team is now investigating this question using their new results on the composition of the stars’ crust.